1. Field of the Invention
This invention relates to a method and apparatus for removing aquatic organisms in a cooling water system used for a tubular heat exchanger such as a condensor, from the cooling water system. More particularly, the present invention relates to a method and apparatus for removing those extraneous matters, which are filtrated onto the outer circumferential surface of a filter element disposed inside a casing, outside the system by means of a turning flow.
2. Description of the Prior Art
A considerable distance exists from an intake to a condensor in a thermal power plant or an atomic power plant using brine as cooling water, and extraneous matters such as small seaweed, infant shellfish, and the like that pass through a screen of the intake grow inside a cooling water pipe. Since these extraneous matters are likely to clog the small cooling water pipes of the condensor, an apparatus for removing the extraneous matters is disposed upstream, and in the vicinity, of the condensor so as to remove them.
Japanese Patent Publication No. 49898/1980, for instance, proposes an aparatus for removing extraneous matters. A blow-down operation, which is carried out to discharge the extraneous matters deposited to a filter element, is started when a pressure difference between the pressure upstream of the filter element and the pressure downstream thereof (hereinafter called "filter element differential pressure") exceeds a predetermined value.
A butterfly wave of the extraneous matter removing apparatus is caused to displace by a predetermined angle in either one of the closing directions so as to generate a turning flow along the outer circumference of the filter element. A blow-down valve is opened simultaneously with the displacement of the butterfly valve to discharge the extraneous matters together with part of the cooling water.
In a system which controls the flow rate of the cooling water to a minimum necessary level in accordance with a turbine load, the filter element differential pressure varies remarkably with the flow rate of the cooling water if the flow rate changes, even though the rate of clogging of the filter element is constant.
The relation between the rate of clogging of the filter element and the filter element differential pressure P in the blow-down operation will be described with reference to FIG. 8.
This graph shows the flow rate of cooling water passing through the extraneous matter removing apparatus for an inlet flow velocity v in the piping between a cooling water pump and the extraneous matter removing apparatus. The apparatus is set so that the blow-down operation occurs when the filter element differential pressure .DELTA.P exceeds a preset value, for example .DELTA.P.sub.2.
The blow-down operation is effected at a rate of clogging of 50% when the inlet flow velocity is v.sub.2, at a rate of 70% when the inlet flow velocity is v.sub.1 at which the flow rate of the cooling water is small, at a rate of 30% when the inlet flow velocity is v.sub.3 at which the flow rate of the cooling water is great, and at a rate of 0% when the inlet flow velocity is v.sub.4 at which the flow rate of the cooling water is extremely great.
If the flow velocity of the cooling water is low, the blow-down operation is not effected even though the filter element gets clogged to a somewhat high extent. When the flow rate of the cooling water is great, the blow-down operation is frequently effected, even though the filter element is hardly clogged, and the cooling water is discharged in vain.
If the blow-down operation is not effected even though the filter element is clogged to a considerable extent, the extraneous matters deposited to the filter element are not scraped off even when the blow-down operation is next carried out, so that the blow-down operation is no longer effective to recover the filter element.
At the start of the blow-down operation, the blow-down valve is fully opened simultaneously with the displacement of the butterfly valve. Because the extraneous matters are discharged while their concentration is low and in addition, because the blow-down valve is kept open for a long period, the cooling water is discharged in vain.